An anti-lock brake system (ABS) is a device that prevents wheels from being locked. There is a known technique, disclosed in, for example, PTL 1, that determines whether the state (concavo-convex road surface) of a traveling road surface is bad in, for example, a motorcycle.
The technique in PTL 1 determines that the road state is bad when the number of times the wheel acceleration becomes equal to or more than a predetermined level (L1) is equal to or more than a predetermined number (KN1).
On the other hand, although there is another technique, disclosed in, for example, PTL 2, that makes appropriate ABS control suited for road surface situations by changing parameters used for ABS control based on the result of bad road determination as in, for example, the technique in PTL 2, it is desirable to perform bad road determination with better precision.